An important development in the field of networks at present is the convergence of voice and data networks. An important future scenario is that data, voice and video information are transmitted via a packet-oriented network, with newly developed network technologies assuring that requirement features for various classes of traffic are observed. Future networks for various types of traffic are anticipated to operate in packet-oriented fashion. Current development activities relate to the transmission of voice information via networks which are conventionally used for data traffic, particularly IP (Internet Protocol) based networks.
To allow voice communication via packet networks and particularly IP based networks in a quality which is equivalent to that of voice transmission via circuit-switched networks, it is necessary for quality parameters such as the delay for data packets or jitter to be kept within narrow limits. In the case of voice transmission, it is of great importance to the quality of the service provided for the delay times not to substantially exceed values of 150 milliseconds. To achieve a correspondingly short delay, work is being carried out on improved routers and routing algorithms which are intended to allow faster handling of the data packets. In the case of routing via IP networks, a distinction is usually drawn between intra-domain and inter-domain routing. Data transmission via the Internet usually involves networks—as well as subnetworks, to domains or what are known as autonomous systems—from various network operators. The network operators are responsible for the routing within the domains which come under their area of responsibility. Within these domains, they have the freedom to adapt the procedure for routing according to their own wishes as desired, just so long as it is possible to comply with quality-of-service features. The situation is different in the case of routing between different domains, where different domain operators are connected to one another. Inter-domain routing is complicated by the fact that first it is necessary to determine the best possible paths to the destination via various domains, but secondly, domain operators are able to apply strategies locally which influence global calculation of optimum paths on the basis of objective criteria. By way of example, one strategy involves domains from network operators in a particular country being avoided for traffic of a certain origin. However, this strategy is now generally not known to all network operators with domains via which the traffic is routed, i.e. a network operator needs to make a local decision regarding the domain to which he forwards traffic without having complete information about the best path in terms of metrics. The strategies are frequently also referred to by the term “policies”.
For the routing between various domains, what are known as Exterior Gateway Protocols EGP are used. At present, version 4 of the Border Gateway Protocol (frequently shortened to BGP), described in more detail in RFC (Request for Comments) 1771, is usually used on the Internet. The Border Gateway Protocol is what is known as a path vector protocol. A BGP entity (the term “BGP speaker” is frequently used in English literature) is informed by its BGP neighbors about possible paths to destinations which can be reached via the respective BGP neighbor. Similarly communicated properties of the paths (path attributes) provide the BGP entity with the best respective path from its local point of view to the destinations which can be reached. The BGP protocol involves four types of messages being exchanged between BGP entities. The messages including what is known as an “update” or change message, which is used to propagate path information through the entire network and which allows the network to be optimized with topology changes. Sending update messages usually results in the path information being adapted on all BGP entities in the network for the purpose of routing optimized in line with the locally available information. In addition, what are known as “keepalive” or “state” confirmation messages are a feature, these being used by a BGP entity to enlighten its BGP neighbors about its operability. In the absence of these messages, the BGP neighbors assume that the link to the BGP entity has been disrupted.
The propagation of topology information using the BGP protocol has the drawback that when there are frequent change indications the load which arises as a result of the messages propagated through the network in order to indicate the change is considerable, and that the network does not converge out if change messages follow in too quick succession. This problem that the network does not converge out or that the inter-domain routing does not become stable, has been addressed by what is known as the route flap damping approach. The idea of this concept is to sanction the indication of a change by a BGP neighbor. When a change message is received, the damping parameter is increased, and change reports are ignored if the damping parameter exceeds a threshold. The damping parameter decreases exponentially over time. Consequently, change reports from BGP entities are ignored so long as the damping value has not dropped below the lower threshold (reuse threshold). However, the method has the drawback that it carries the risk of a potential loss of connection, which cannot be tolerated for realtime traffic.
EP 1453250, which is incorporated by reference in its entirety herein, describes an approach for extending the BGP protocol by a method for rapid reaction to link failures in the case of Inter-domain routing. This approach provides alternative paths, with no prior propagation of change messages for the entire network being required. A change to the routing is made only along alternative paths. This limited adjustment to the routing allows a rapid reaction to faults. In the case of prolonged faults (persistent error), it is additionally possible to perform topology adaptation in the network using the BGP protocol.